Method of manufacturing seamless steel pipes

ABSTRACT

In a method of manufacturing a seamless steel pipe by piercing a blank by means of a piercing machine and then rolling the pierced blank into the product by means of a rolling mill, the blank is pierced at a reduction rate of 75 to 85 percent by using a piercing machine comprising rolls having flat gorges at the center thereof and a piercing plug provided with an annular recess on its outer periphery at portions corresponding to the gorges and then the pierced blank is rolled by a single pass and at a reduction rate of 15 to 25 percent by means of a rolling mill having rolls and a plug, the curvature of the relieve of the caliber of the rolls of the rolling mill is reduced to minimize the tendency of forming fins on the surface of the product.

United States Patent N agai et al.

[54] METHOD OF MANUFACTURING SEAMLESS STEEL PIPES [72] Inventors: Hiroshi Nagai; Haruo Kamimura,

both of Kawasaki, Japan [73] Assignee: Nippon Kokan Kabushika Kaisha Kawasaki, Japan [22] Filed: Oct. 20, 1970 [21] Appl. No.: 82,258

[52] US. Cl ..72/97, 72/209 [51] Int. Cl. ..B2lb 19/04 [58] Field of Search ..72/97, 209, 283

[56] References Cited UNITED STATES PATENTS 792,231 6/1905 Nicholson ..72/97 2,176,155 10/1939 Stiefel ..72/97 2,264,455 12/1941 Peck ..72/97 2,274,579 2/ 1942 Bannister et a1 ..72/97 [451 Oct. 31, 1972 Primary Examiner-Richard J. Herbst Attorney-Flynn & Frishauf [57] ABSTRACT In a method of manufacturing a seamless steel pipe by piercing a blank by means of a piercing machine and then rolling the pierced blank into the product by means of a rolling mill, the blank is pierced at a reduction rate of 75 to 85 percent by using a piercing machine comprising rolls having flat gorges at the center thereof and a piercing plug provided with an annular recess on its outer periphery at portions corresponding to the gorges and then the pierced blank is rolled by a single pass and at a reduction rate of 15 to 25 percent by means of a rolling mill having rolls and a plug, the curvature of the relieve of the caliber of the rolls of the rolling mill is reduced to minimize the tendency of forming fins on the surface of the product.

1 Claim, 8 Drawing Figures PlTENTEnnmal 1912 3,701 272 sum 1 or 3 VIII Ila /AYIIIIZ PATENTED um 3 1 m2 SHEET 2 OF 3 FIG. 6

FIG. 7

PATENTED 3.701, 272

sum 3 or 3 FIG. 8

METHOD OF MANUFACTURING SEAMLESS STEEL PIPES BACKGROUND OF THE INVENTION This invention relates to a method of manufacturing seamless steel pipes by means of Mannessman plug mill wherein the reduction rate of the piercing machine is increased to produce a thin walled pierced blank by improving the configuration of the gorge of the rolls of the piercing machine and by improving the construction of the piercing plug. The pierced blank is thin rolled by means of a rolling mill at a reduced reduction rate to minimize the tendency of forming fins on the outer surface of the product, thus finishing the rolling operation by a single pass.

The efficiency of manufacturing seamless steel pipes by using a Mannessman plug mill is determined mainly by the cycle time of a line including a piercing machine, a rolling mill and a pipe polishing machine. The rolling mill used in the prior art installation for manufacturing steel pipes generally requires two, three or more passes, thus decreasing the efficiency of rolling. This is because, when manufacturing seamless steel pipes by the Mannessman plug mill, it has been considered that to decrease defects or stepped configuration on the inner and outer surfaces of the products, it is necessary to pierce the blanks to produce products having as far as possible thick walls and to pierce the blanks at low speeds whereas in rolling mills, in order to prevent plug marks, damage to the rolls and to prevent the tendency of forming fins on the products, it has been considered that it is necessary to subject the pierced blanks to light reduction and to perform two or more passes.

In the pipe polishing machine, a particular pass schedule and reduction rate have been used in order to eliminate equal wall thickness of the products and to obtain smooth surface finish. It has also been thought that there is a limit on the height of fins on the rolled blanks that can be eliminated by the pipe polishing machine.

SUMMARY OF THE INVENTION It is the principal object of this invention to reduce the number of passes of the pierced blank through the rolling mill to only one pass.

Another object of this invention is to provide a product having a smooth surface finish free from fins or surface irregularities.

According to this invention, these and other objects can be attained by improving the construction of the piercing rolls and the piercing plug of the piercing machine and by reducing the curvature of the relieve of the caliber of the rolls of the rolling mill.

Briefly stated, according to this invention, in a method of manufacturing a seamless steel pipe by piercing a blank by means of a piercing machine and then rolling the pierced blank into the product by means of a rolling mill, the blank is pierced at a reduction rate of 75 to 85 percent by using a piercing machine comprising rolls having flat gorges at the center thereof and a piercing plug provided with an annular recess on its outer periphery at portions corresponding to the gorges and then the pierced blank is rolled by a single pass and at a reduction rate of to percent by means of a rolling mill having rolls and a plug. The radius of the relieve of the caliber of the rolls of the rolling mill is reduced to minimize the tendency of forming fins oh the surface of the product.

By flattening the piercing rolls at the gorge and by providing the annular groove on the piercing plug, it is possibleto pierce the blank at a higher reduction rate than by the conventional piercing machine without forming stepped portions on the inner and outer surfaces of the pierced blank. This also permits small reduction rate of the rolling mill. Further, the improvement of the construction of the caliber of the rolls of the rolling mill as well as the above described lesser reduction rate effectively prevents the tendency of forming fins on the outer surface of the product thus improving the surface finish thereof.

BRIEF DESCRIPTION OF THE DRAWINGS In the accompanying drawings:

FIG. 1 is a side view, partly in section, of a pair of rolls and a blank, to explain the operation of a piercing machine;

FIG. 2 shows a cross-section taken along a line II II in FIG. 1;

FIG. 3 is an enlarged view to show the relationship between the piercing rolls and the piercing plug;

FIG. 4 is a side view, partly in section, to explain a rolling operation;

FIG. 5 is a partial side view of a mill caliber;

FIGS. 6 and 7 are plots of reduction rates varying in the harmonic progression along the entire contact length of the piercing plug, wherein FIG. 6 shows the prior art whereas FIG. 7 this invention; and

FIG. 8 shows the detail of the piercing plug shown in FIG. 3 wherein the dotted lines show the contour of the piercing plug utilized in the prior method whereas solid lines show the contour of the piercing plug utilized in this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT As diagrammatically shown in FIGS. 1 and 2, a typical piercing machine comprises piercing rolls 1, a piercing plug 2 and guide shoes 3 for guiding a steel blank 4. According to the conventional design, the gorges 5 at the center of piercing rolls 1 are pointed to force the blank to the rear side against the force applied by the plug, whereas according to this invention, gorges are not pointed but are made flat and the portions of the piercing plug confronting against the gorges are depressed around the entire periphery to form an annular recess 6 as best shown in FIG. 3. This annular recess is shaped as shown in FIG. 8 to prevent irregularities in the inner and outer surfaces of the pierced blank due to the difference in the reduction rates of the products produced by the gorges of the piercing rolls and the piercing plug having a curved surface corresponding to the gorges. Since the prior plug is not provided with such annular recess, as shown in FIG. 6 many surface irregularities'are formed on both the inner and outer surfaces at portions remote from the end of the plug by 60 to mm. According to this invention, the percentage reduction is made uniform as shown in FIG. 7 by providing the annular recess for the piercing plug. This also makes it possible to prevent edgescratches caused by the gorges even when a large rolling pressure is applied for the purpose of heavily reducing the wall thickness of the product. In this manner, the blank 4 is smoothly pierced by the cooperation of the piercing rolls 1 and the piercing plug 2, thus reducing scratches on the inner and outer surfaces of the products. Near the rear end of the piercing plug 2, the blank is subjected to ironing action by the piercing rolls 1 and the piercing plug 1 thus preventing stepped configuration from being formed on the inner and outer surfaces of the pierced blank. In this manner, as the blank is rolled in the piercing machine, the reduction rate is increased to 75 to 85 percent over the prior art reduction rate of about 70 percent.

Where the pierced blank is reduced by a rolling mill comprising rolls 7 and a plug 8, the blank 9 is reduced while it is subjected to a heavy reduction as shown in F IG. 4. The end of the plug 8 is shaped as a frustum of a cone. As shown in FIG. 5, the radius R of the relieve I 1 of the rolling caliber is made smaller than that R of the prior construction to prevent the tendency of forming fins on the rolled product. This construction improves grippingthus enabling heavy reduction of a blank of thick wall. According to the conventional method, the reduction rate is about 40 percent, whereas in this invention it is reduced to 15 to 25 percent.

With the improved configurations of the roll and plug; it becomes possible to perform a portion of the required reduction by the piercing machine whereby the blank can be subjected to heavy reduction without the fear of forrning fins on the product. In other words, it is possible to manufacture pipes by a single pass of heavy reduction, with the result that the rolling efficiency (ton/hour) can be increased by 150 to 180 percent over the prior method. Moreover, as the desired product can be produced with a single pass, it is possible to reduce the time and labor required for exchanging plugs, and can improve the quality and yield of the product, thus eliminating every defects of the prior method.

The invention can be applied to all types of steels and various dimensions of the seamless pipes that can be rolled by the Mannessman plug mill.

As above described according to this invention, it is possible to provide a reduction rate of 75 to 85 percent by the piercing machine and a reduction rate of 15 to 25 percent by the rolling mill.

Following examples compare these data with those of the prior art.

EXAMPLE 1 Blanks of a dimension of 104.5 Kg/m were formed into seamless pipes according to this invention and to the prior method, the dimension of the products being 15.00 Kg/m for both cases.

TABLE 1 Prior Method This Invention Blank 104.5 Kg/rn 104.5 Kg/m Reduction rate 25.20 Kg/m 20.80 Kgjm by piercing plug 75.9 80.1 Reduction rate 15.00 Kg/m 15.00 Kg/m by rolling mill 40.5 27.9

EXAMPLE 2 Following table 2 compares the reduction rate by the piercing plug and the reduction rate by the rolling mill of the products of 19.0 Kg/m manufactured from steel 1. In a method of manufacturing a seamless steel pipe by piercing a blank by means of a piercing machine and then rolling the pierced blank into the product by means of a rolling mill, the improvement which comprises the steps of piercing said blank at a reduction rate of to percent by using a piercing machine comprising rolls having flat gorges at the center thereof and a piercing plug provided with an annular recess on its outer periphery at portions corresponding to said gorges, and rolling the pierced blank by a single pass and at a reduction rate of 15 to 25 percent by means of a rolling mill having rolls and a plug, the radius of the relieve of the caliber of the rolls of said rolling mill being reduced to minimize the tendency of forming fins on the surface of the product. 

1. In a method of manufacturing a seamless steel pipe by piercing a blank by means of a piercing machine and then rolling the pierced blank into the product by means of a rolling mill, the improvement which comprises the steps of piercing said blank at a reduction rate of 75 to 85 percent by using a piercing machine comprising rolls having flat gorges at the center thereof and a piercing plug provided with an annular recess on its outer periphery at portions corresponding to said gorges, and rolling the pierced blank by a single pass and at a reduction rate of 15 to 25 percent by means of a rolling mill having rolls and a plug, the radius of the relieve of the caliber of the rolls of said rolling mill being reduced to minimize the tendency of forming fins on the surface of the product. 